Which 12ax7 socket is the phase inverter on a DT50?

I'm replacing the preamp tubes in my DT50 112. I believe one is an input buffer and the other is a phase inverter. I ordered a standard 12ax7 for the input buffer, but I ordered a balanced 12ax7 for the phase inverter. The sockets are labeled V3 and V4 with the power amp tubes labeled V1 and V2. Conventional wisdom would say that V3 is the power inverter because the socket number is next in line to the power tubes. However the only thing I could find online is someone who "thought" the numbering system was different on DT50s. So, does anyone know for sure which socket is the phase inverter? Thx!!

I'm replacing the preamp tubes in my DT50 112. I believe one is an input buffer and the other is a phase inverter. I ordered a standard 12ax7 for the input buffer, but I ordered a balanced 12ax7 for the phase inverter. The sockets are labeled V3 and V4 with the power amp tubes labeled V1 and V2. Conventional wisdom would say that V3 is the power inverter because the socket number is next in line to the power tubes. However the only thing I could find online is someone who "thought" the numbering system was different on DT50s. So, does anyone know for sure which socket is the phase inverter? Thx!!

Did you find this to be correct? V4 is the inverter? I did the exact same thing with my dt50-112, buying the balanced pi and then realizing i don't know which socket...

i just tried the mullard in the pi socket and noise floor seems to be cut in half. next up: mullard in the boost socket with original eh back in its pi socket. then on to my final (i hope) test with both new tubes in place.

hard to say...I know a 12AX7 used as an inverter runs higher current thru both triodes...I suspect the trace matching is a bit different than for a 12AX7 intended for a buffer...No idea how that would translate...perhaps slightly less output potential than one intended for preamp use....

In the case of the DT50 12AX7 buffer, only one triode is used...So I have been debating on sticking one of these on my SPAX7 since the other triode has never really been used...I don't know about the "vibration reducing base"...but it would be cool to get the full life out of the buffer tube...

hard to say...I know a 12AX7 used as an inverter runs higher current thru both triodes...I suspect the trace matching is a bit different than for a 12AX7 intended for a buffer...No idea how that would translate...perhaps slightly less output potential than one intended for preamp use....

In the case of the DT50 12AX7 buffer, only one triode is used...So I have been debating on sticking one of these on my SPAX7 since the other triode has never really been used...I don't know about the "vibration reducing base"...but it would be cool to get the full life out of the buffer tube...

Disappointingly, I've been hearing a slight 60 cycle hum beginning with test 2, and even going back to test 1 config does not abate. (even with no guitar cord plugged in) The question now is: Was the hum always there, but I just couldn't hear it before through the whoosh of the noise floor? (which is now much improved) -- I believe I'll swap back the original tubes tomorrow--for tonight enough of that has taken place already.

Sorry I don't totally follow you... What is SPAX7? Is that equivalent to a SAG-AX7? So you're going to try a matched PI tube in the boost socket (which is also called a buffer socket)?

Sorry....SPAX7 is Mesa's best 12AX7...I use that as a buffer...I use the GTSAG as the inverter...I did compare them both as inverters and found no difference...Never put the GTSAG in as a buffer...It's not traced for that...would not hurt anything...just not measured for that usage...

Same as always, I'm plugged into the same power strip plugged into the same outlet. I tried the ground lift switch and that made absolutely no difference.

I did have some buzz issues before when I was running HD500 studio direct (no amp), though intermittently. The source of that was a surface mount failure inside the HD. This bizzare component the size of a caramel square eventually fell right off the board, enabling me to isolate and correct the problem. I think I'll try disconnecting the POD and XLR cable and see if that changes anything. Any other ideas? The timing of my noticing this seems more than coincidental to me...

perhaps a structured approach would help you isolate your hum issue...sounds like you have too many variables to quantify...

Run the amp by itself...Nothing connected other than power and the speaker...Does it hum?

if so, try to isolate the modes of the amp....

Does it hum in Class AB?

Try each NFL and each mode...

Does it hum in Class A?

Try each NFL and each mode...

Basically you have 16 configuration combinations of the power amp...

If the amp is pretty quiet, then you know the the issue is probably elsewhere...now plug in your guitar to the amp and see what the noise deal really is with your guitar and the amp itself...then add you HD500....maybe you need an AVR or a decent power conditioner? have you ever measured your house power? Do you know for a fact that you have zero voltage on ground? Are there CDL bulbs or light tubes nearby? CRT monitor...computer?

perhaps a structured approach would help you isolate your hum issue...sounds like you have too many variables to quantify...

Run the amp by itself...Nothing connected other than power and the speaker...Does it hum?

if so, try to isolate the modes of the amp....

Does it hum in Class AB?

Try each NFL and each mode...

Does it hum in Class A?

Try each NFL and each mode...

Basically you have 16 configuration combinations of the power amp...

If the amp is pretty quiet, then you know the the issue is probably elsewhere...now plug in your guitar to the amp and see what the noise deal really is with your guitar and the amp itself...then add you HD500....maybe you need an AVR or a decent power conditioner? have you ever measured your house power? Do you know for a fact that you have zero voltage on ground? Are there CDL bulbs or light tubes nearby? CRT monitor...computer?

perhaps a structured approach would help you isolate your hum issue...sounds like you have too many variables to quantify...

Run the amp by itself...Nothing connected other than power and the speaker...Does it hum?

if so, try to isolate the modes of the amp....

Does it hum in Class AB?

Try each NFL and each mode...

Does it hum in Class A?

Try each NFL and each mode...

Basically you have 16 configuration combinations of the power amp...

If the amp is pretty quiet, then you know the the issue is probably elsewhere...now plug in your guitar to the amp and see what the noise deal really is with your guitar and the amp itself...then add you HD500....maybe you need an AVR or a decent power conditioner? have you ever measured your house power? Do you know for a fact that you have zero voltage on ground? Are there CDL bulbs or light tubes nearby? CRT monitor...computer?

My DT50-112 buzzes in all voices, classes, modes, and channels--everything except standby mode. I've tested each and every combination as you prescribed, and here are the breakdowns/patterns it follows: It buzzes worse in class A than AB. It buzzes worse on pentode than triode. The buzz follows the noise floor pattern through the voices, i.e. I is lowest,, louder in II, louder still in III, and in IV it gets quieter and changes in tone, which I believe to be due to harmonics but can't vouch.

I checked my line voltage, and ground on one leg is about 4V different than the cold side. For the other leg it's only .7V so I plugged into that outlet. No difference. No difference with ground lift either, as was the case yesterday, so this entire paragraph isn't surprising.

I do not believe power conditioners should be necessary in buildings built after 1975 because electronic devices are supposed to come with adequate filtration and surge protection. I have numerous amplifiers and other devices in my house, and only one has a ground buzz issue. I believe this to be a defect in the amp and I'm crushed.

My DT50-112 buzzes in all voices, classes, modes, and channels--everything except standby mode. I've tested each and every combination as you prescribed, and here are the breakdowns/patterns it follows: It buzzes worse in class A than AB. It buzzes worse on pentode than triode. The buzz follows the noise floor pattern through the voices, i.e. I is lowest,, louder in II, louder still in III, and in IV it gets quieter and changes in tone, which I believe to be due to harmonics but can't vouch.

I checked my line voltage, and ground on one leg is about 4V different than the cold side. For the other leg it's only .7V so I plugged into that outlet. No difference. No difference with ground lift either, as was the case yesterday, so this entire paragraph isn't surprising.

I do not believe power conditioners should be necessary in buildings built after 1975 because electronic devices are supposed to come with adequate filtration and surge protection. I have numerous amplifiers and other devices in my house, and only one has a ground buzz issue. I believe this to be a defect in the amp and I'm crushed.

Perhaps you should get it serviced...

Not sure what to say about your power belief other than it's a free country...Sorry I could not help you...

Not sure what to say about your power belief other than it's a free country...Sorry I could not help you...

I really appreciate your help! I'm in the process of searching for a service center now. I suppose that if for some reason my buzz is normal to the amp and the only way I can get rid of the buzz is with external power conditioning, the tone of this amp is so awesome that I'd bend the rules and do whatever it takes. However, I doubt it's normal as I'm not reading about this issue with other DTs and also because the buzz is new rather than something that was always there. I'm thinking its some kind of solder or PCB trace issue. Hopefully the tech will find it soon!!!

Not sure what to say about your power belief other than it's a free country...Sorry I could not help you...

Service centers will keep my amp anywhere from 2 weeks to 2 months, I found out. When I saw that GC was having a sale on a rackmount power conditioner at $30 (list is $100) I thought "what the heck?" But when I plugged into it, the AC noise actually got worse! More of a buzz than a hum, with a volume increase to boot. My guess here is that in conditioning the power, filtering out RFI and such, the harmonic content of the 60-cycle AC waveform is being altered. In my case the device actually dirtied it up relative to what's at the outlet. I think I'll try the comparable Furman unit, but if that doesn't fix it...

Service centers will keep my amp anywhere from 2 weeks to 2 months, I found out. When I saw that GC was having a sale on a rackmount power conditioner at $30 (list is $100) I thought "what the heck?" But when I plugged into it, the AC noise actually got worse! More of a buzz than a hum, with a volume increase to boot. My guess here is that in conditioning the power, filtering out RFI and such, the harmonic content of the 60-cycle AC waveform is being altered. In my case the device actually dirtied it up relative to what's at the outlet. I think I'll try the comparable Furman unit, but if that doesn't fix it...

Wow, bummer! Your DT amp is really putting you through the ringer. It sounds like in the long run, you will be happiest / most confident in the gear if you send it the service center, or to Line6, and wait it out. That way, you will *know* that it's performing the way it should.

While you are waiting for the amp to get back, work on those studio/direct 'backup' patches, for just in case you ever need them on stage!

Wow, bummer! Your DT amp is really putting you through the ringer. It sounds like in the long run, you will be happiest / most confident in the gear if you send it the service center, or to Line6, and wait it out. That way, you will *know* that it's performing the way it should.

While you are waiting for the amp to get back, work on those studio/direct 'backup' patches, for just in case you ever need them on stage!

Right on! Now that i've been working with preamp patches for a bit, my full amp patches will probably be tweaked all over again when i go back to studio direct...